Place potatoes in large 6-7 quart slow cooker or crock pot. Pour in water. Sprinkle salt over potatoes.Place lid on slow cooker and set on high heat. Don't lift the lid! Cook for about 4 hours.Leaving the insert (bowl) in the slowcooker do the following:Do not drain the potatoes. Place the butter in with the potatoes. Using a potato masher, mash the potatoes and butter together. Add the half and half (about 1½ cups). Continue to mash until desired consistency. Add the remaining ½ cup of half and half, if needed. Add sour cream (if using) and mix well. Taste and add more salt and a little pepper if desired.Turn the heat to warm and keep warm until ready to serve for up to two hours. If needed, just before serving, add additional milk or half and half and fold the potatoes